Commercial alkaline battery systems, for example iron/nickel batteries or iron/air batteries are employing iron electrodes to an increasing degree. The iron/nickel system is distinguished by particularly high energy content and low weight with great power capability. By comparison with batteries which use zinc as the negative electrode, these systems give increased operational reliability, and longer life. By comparison, for example, with a cadmium electrode, the iron electrode is substantially cheaper, readily available and is non-toxic. However, commonly used iron electrodes exhibit certain insufficiencies and negative characteristics. Among these are a relatively high degree of self-discharge and a tendency to passivate, especially at high charge and discharge current densities as well as at low temperatures.
For example, iron electrodes whose active mass consists of very pure carbonyl iron become passivated after only a few cycles and thus inoperative. The causes for this passivation are not entirely clear but it is assumed that trivalent iron oxide compounds which are difficult to reduce accumulate on the surface of the active mass of the electrode and inhibit the charging process.
It is known in the art to employ sulfur to counteract the passivating tendency of the electrode. Sometimes the sulfur is already contained in trace amounts in the material during manufacture and at other times it is added to the electrode material during manufacture.
Known in the art are methods in which sulfur or sulfides such as iron sulfide, cadmium sulfide, mercury sulfide, or indium sulfide are added to the active electrode mass. It has also been found that some inorganic an organic sulfur-containing combinations with the electrolyte tend to counteract the passivating effect. The active sulfur travels to the active electrode through the electrolyte.
The concentration of sulfur which has been proposed lies in the range of 0.05-5.0 percent of the weight of the iron in the active mass of the electrode. Selenium and tellurium as well as their compounds have also been mentioned as having a positive anti-passivating effect. Recently lead sulfide has been mentioned as an additive to counteract electrode passivation.
It is the common disadvantage of all these additives that they are unable to guarantee a lasting activation of the iron electrode in long-term operation, especially when the electrode is subjected to high charging and discharging current densities and in particular when the battery is overcharged. It is the property of sulfur in the form of sulfide dissolved in an alkaline electrolyte to be adsorbed at the iron electrode. A majority of the sulfide ions will also diffuse from the electrode through its pores and come in contact with oxygen dissolved in the electrolyte or even with the positive electrode. Thus, an oxidation to produce sulfate ions which are ineffective to activate the electrode is virtually unavoidable. Sulfate is virtually incapable of reduction to effective sulfide ions due to kinetic barriers. Instead, it precipitates as alkali sulfate and can even plug up the pores of the electrode. The use of high sulfide concentrations does not result in any substantial prolongation of the lifetime of the electrode.
The effect of a loss of sulfide is generally the gradual decrease of the battery capacity. In iron/nickel cells, the effect may be masked for a long time if the iron electrodes are of substantially greater capacity than the positive electrodes. However, the effect becomes very noticeable when the battery is deep-discharged at high current densities. On the other hand, an activation of the active iron mass requires available sulfide. Thus, the normal battery operation is necessarily impeded because several careful and conservative charging and discharging processes are required to restore the iron electrode to the normal capacity. In some cases, it may be necessary to exchange the electrolyte which introduces additional cost and time losses.
It has been determined that the long-lasting activation of the active iron mass requires the maintenance of a given sulfide concentration in the electrolyte of the porous electrode. This condition has not heretofore been achieved but is attained for the first time by the provisions of the present invention.
The sulfur, selenium and tellurium compounds used heretofore place sulfide ions in the electrolyte not only due to chemical dissolution, but also because of the electrochemical decomposition on the iron electrode during the charging process. As a consequence, there occur unnecessarily high sulfide concentrations, a rapid migration and oxidation to sulfate and a rapid depletion of the sulfide supply. Any uncontrolled and extensive overcharge can produce a local sulfide concentration which is so high as to block the active mass. It has been found that when the sulfide concentration exceeds 10.sup.-2 -10.sup.-1 mol/l at 20.degree. C., the activating effect is actually reversed. The result is a very reduced effectiveness of the active mass and a reduced power capability of the electrode. Even at high operating temperatures of up to 70.degree., the aforementioned concentration of sulfides should not be exceeded. In that case, the reverse effect will occur when the cell cools off while the high sulfide concentration is temporarily metastable.
Another disadvantage of the known art is that the electrochemical decomposition of the metal sulfides, selenides and tellurides causes deposition of metal on the negative electrode and may lead to malfunctions.